A wide variety of automotive and industrial internal combustion engines routinely rely upon a synchronous belt drive system to transmit torque from a driver sprocket on a crankshaft to a driven sprocket such as on a camshaft. Camshafts produce vibrations which can be detrimental to the operating life of the engine and synchronous belt. In particular, actuation of the intake and exhaust valves though contact with the camshaft lobes causes a cyclic fluctuating torque load to be transmitted through the synchronous belt.
Prior art attempts to attenuate the fluctuating torque loads include use of camshaft dampers as well as damped belt tensioners.
Teachings disclose the use of non-circular (oval) sprockets to control strongly fluctuating torques, for example, VDI Progress Reports No. 272, “Non-Uniform Transmission Belt Drives” by Dipl.-Ing. Egbert Frenke.
Other attempts include use of a rotor having an oval non-circular profile having at least two protruding alternating with receding portions. The rotary load assembly presents a periodic fluctuating load torque when driven in rotation in which the angular positions of the protruding and receding portions of the non-circular profile relative to the angular position of the second rotor, and their magnitude, substantially cancels the alternating belt tensions caused by fluctuating load torque of the rotating assembly.
Representative of the art is U.S. Pat. No. 7,044,875 which discloses a synchronous drive apparatus and method, wherein the apparatus comprises a plurality of rotors comprising at least a first and a second rotor. The first rotor has a plurality of teeth for engaging the engaging sections of an elongate drive structure, and the second rotor has a plurality of teeth for engaging the engaging section of the elongate drive structure. A rotary load assembly is coupled to the second rotor. The elongate drive structure engages about the first and second rotors. The first rotor is arranged to drive the elongate drive structure and the second rotor is arranged to be driven by the elongate drive structure. One of the rotors has a non-circular profile having at least two protruding portions alternating with receding portions. The rotary load assembly is such as to present a periodic fluctuating load torque when driven in rotation, in which the angular positions of the protruding and receding portions of the non-circular profile relative to the angular position of the second rotor, and the magnitude of the eccentricity of the non-circular profile, are such that the non-circular profile applies to the second rotor an opposing fluctuating corrective torque which reduces or substantially cancels the fluctuating load torque of the rotary load assembly.
What is needed is a belt drive sprocket system comprising an obround sprocket having a toothed surface and at least one linear portion disposed between two circular portions, the circular portions having a constant radius, the linear portion having a length which relates to a torque fluctuation amplitude.